Typically, viscosity is a constant, or a coefficient of fluid and represents the viscous resistance of the fluid. Numerous viscometers have been developed and put into use for the purpose of measuring this coefficient.
In 1953, Greenspan and Wimenitz developed a viscometer to measure the viscosity of a gas, by attaching and facing two Helmholtz resonators towards each other (M. Greenspan and F. N. Wimenitz, “An Acoustic Viscometer for Gases I”, NBS Report 2658 (1953)). However, the Greenspan viscometer is relatively large, and displays a 38% margin of error. Thus, the Greenspan viscometer was not considered an ideal solution.
In 1996, K. A. Gillis offered a more precise viscometer than the Greenspan viscometer. A process of experimental error and correction was applied to the design, reducing the margin of error of the viscometer to ±0.5%, which resulted in more accurate gas viscosity (Refer to R. A. Aziz, A. R. Janzen, and M. R. Moldover, Phys. Rev. Lett. 74, 1586 (1995)).
However, in this method, the valid frequency interval is limited to low frequency regions. Gillis' viscometer could only be applied to frequencies under 200 hz. This was due to the fact that the Helmholtz resonator was applied assuming that the product of the wavenumber of the sound wave and the characteristic length was less than 1. In addition, the Gillis viscometer did not take liquid viscosity measurements; only gas viscosity was measured, and the large amount of fluid needed to measure said viscosity made the Gillis viscometer impractical.
In yet another approach, U.S. Pat. No. 6,141,625 discloses a viscosity module with a crystal resonant sensor. It is a portable viscometer capable of measuring viscosity using only a small amount of reagent fluid; viscosity is measured using thin, disc-shaped crystal films.
The resonant frequency of these crystal films is obtained by operating the sensor in thickness shear mode. To do so, a signal is passed through the film, by positioning electrodes on the top and bottom of the crystal film. If a liquid is present on the top portion of the film, the resulting power loss of the signal traveling through an additional barrier will cause damping in the crystal film's resonant frequency. Thus, the viscosity of the liquid is measured by observing the value of damping in the crystal film's resonant frequency.
The issue with the crystal film viscometer, however, is that the viscometer's crystal sensor must be situated horizontally, with a relatively large amount and even distribution of liquid atop the crystal film. This type of viscometer needs several ml of liquid, assuming that the volume of a single water drop is 0.04 ml. It is also impossible to take viscosity measurements with a single drop of liquid. Since this viscometer depends on gravity applied to liquids, measuring gas viscosity is impossible as well.
On the other hand, there are many viscometers which use capillary tubes to measure viscosities. However, most of these viscometers rely on the differential head caused by gravity to take their measurements. Such viscometers are disclosed in U.S. Pat. Nos. 6,322,624, 6,402,703, 6,428,488, 6,571,608, 6,624,435, 6,732,573, 5,257,529, etc. This reliance on the differential head due to gravity means that such viscometers can only measure liquids, not gases. In addition, such viscometers also require large amounts of liquid, ranging from a few dozen to hundreds of ml.